Active studies and developments have been performed for a planar lightwave circuit (hereinafter referred to as a PLC) that includes a silica based waveguide formed on a silicon substrate. An arrayed waveguide grating (hereinafter referred to as an AWG), a product of the PLC technology, plays an important role in an optical communication system. The AWG is an optical wavelength multiplexing/demultiplexing circuit that has a function for demultiplexing signal light (wavelength multiplexed signal) obtained by multiplexing a plurality of optical frequencies, and providing individual signal lights arranged for predetermined optical frequency channel spacing, or multiplexing the individual signal lights to obtain a single wavelength multiplexed signal.
In accordance with the development of an optical communication system, building of a network system has also begun, wherein multiple points are connected using a ring network, or a mesh network, and communication paths are flexibly changed. For such a high-level network, it has been requested that optical signals be passed through multiple points unchanged, without having to be converted into electric signals. Therefore, an optical wavelength multiplexing/demultiplexing circuit such as is described herein, should have a wide, flat passband and exhibit low-loss transmission. As an optical wavelength multiplexing/demultiplexing circuit having a superior transmission property, proposed in patent document 1 is a synchronized AWG type of optical wavelength multiplexing/demultiplexing circuit that employs both an interference circuit and an AWG. This synchronized type of optical wavelength multiplexing/demultiplexing circuit is characterized in that when an optical signal repetitively passes through a plurality of optical wavelength multiplexing/demultiplexing circuits, there is little deterioration of the optical signal, or a loss is changed little, relative to a fluctuation in the wavelength of an optical signal.
FIG. 23 is a plan view of an example arrangement employed for a conventional synchronized AWG optical wavelength multiplexing/demultiplexing circuit. An optical wavelength multiplexing/demultiplexing circuit 3100 includes a first slab waveguide 3101, arrayed waveguides 3102, a second slab waveguide 3103, second input and output waveguides 3104 and a first input and output waveguide 3105. An optical splitter 3106, a first arm waveguide 3107, a second arm waveguide 3108 and an optical mode converter 3109 are sequentially connected between the first input and output waveguide 3105 and the first slab waveguide 3101. The individual components located between the first input and output waveguide 3105 and the first slab waveguide 3101 constitute an interference circuit.
A synchronized AWG optical wavelength multiplexing/demultiplexing circuit having the above arrangement performs the following operation. Light waves having a plurality of wavelengths enter the first input and output waveguide 3105, and thereafter, they are split by the optical splitter 3106 and are guided to the first arm wavelength guide 3107 and the second arm waveguide 3108. Thereafter, the light waves are propagated as fundamental mode light along the two arm waveguides 3107 and 3108. As a result, because the two arm waveguides have different optical path lengths, a phase difference is generated between the split light waves according to the wavelengths. Thereafter, the separated light waves are again merged at the optical mode converter 3109.
At this time, the fundamental mode that entered the optical mode converter 3109 via the first arm waveguide 3107 is converted into the 1st mode. However, the other fundamental mode light that entered the optical mode converter 3109 via the second arm waveguide 3108 is merged unchanged. Therefore, the light wave output by the optical mode converter 3109 is light provided by the coupling of the fundamental mode light and the 1st mode. The field property of the combined light is altered according to the phase difference between the fundamental mode light and the 1st mode light, i.e., the wavelength of the light.
FIG. 24 is a diagram illustrating an example structure for the vicinity of the optical mode converter of the above described optical wavelength multiplexing/demultiplexing circuit. The optical mode converter 3109 is provided using a directional coupler that includes waveguides that are asymmetrical in width. A waveguide 3109a and a waveguide 3109b are respectively connected to the first arm waveguide 3107 and the second arm waveguide 3108. When the individual waveguide widths are set so as to almost match the effective refractive index of the fundamental mode light passed through the waveguide 3109a, and the effective refractive index of the 1st mode light passed through the waveguide 3109b, the optical mode converter 3109 serves as an optical mode converter for the fundamental mode light and the 1st mode light.
Further, multimode waveguides 3201 and 3203 are sequentially connected to the waveguides 3109b. A tapered waveguide 3202 is connected between the two multimode waveguides 3201 and 3203. However, these waveguides 3201 and 3202 are not requisite components, and are arranged in a case wherein adjustment is required for the widths of the waveguides that are to be connected to the first slab waveguide 3101. Furthermore, the multimode waveguides 3201 and 3203 and the tapered waveguide 3202 should at least enable the propagation of the fundamental mode and the 1st mode. At the terminal end (p axis) of the multimode waveguide 3203 that is connected to the first slab waveguide 3101, the optical field is periodically changed according to a phase difference (a wavelength), and accordingly, the position of the peak of the optical field obtained by light coupling is periodically changed along the p axis.
As described above, an interference circuit arranged between the first input and output waveguide 3105 and the first slab waveguide 3101 transmits, to the first slab waveguide 3101, a light wave that periodically changes the peak position of the optical field according to a wavelength.
Thereafter, based on a difference in an optical path length between waveguides that are adjacent to each other among the arrayed waveguides 3102, a phase difference consonant with a waveform is provided for the light wave that entered the first slab waveguide 3101. Then, the focusing position of the light wave at the terminal end of the second slab waveguide 3103 is changed according to the phase difference (i.e., the wavelength of the light wave that is input). That is, a light wave having a desired wavelength is divided between the second input and output waveguides 3104 that correspond to the focusing position at the terminal end of the second slab waveguide 3103.
For the above described optical wavelength multiplexing/demultiplexing circuit, when the peak location of the optical field is changed at the terminal end of the multimode waveguide 3203, the location at which the light wave enters the first slab waveguide 3101 is also changed. And when the location of the entry for the first slab waveguide 3101 has been changed, the lengths of the optical paths leading to the individual waveguides in the arrayed waveguides 3102 are changed. That is, when a difference in an optical path length is not changed between the adjacent waveguides in the arrayed waveguides 3102, a change occurs in a difference in an optical path length for the entire optical wavelength multiplexing/demultiplexing circuit 3100. As a result, the focusing position for light is changed at the terminal end of the second slab waveguide 3103.
The process sequence performed by the interference circuit described above and by the entire AWG indicates that a difference in an optical path length between the first arm waveguide 3107 and the second arm waveguide 3108 can be employed to adjust the focusing position of a light wave at the terminal end of the second slab waveguide 3103. Furthermore, the parameters for the AWG and the interference circuit on the first slab waveguide side may be set, so that in a specific waveform region, for example, a change in the peak position of the optical field at the terminal end of the multimode waveguide 3203 is synchronized with a change in the focusing position of light at the terminal end of the second slab waveguide 3103, which occurs due to a difference in an optical path length between the adjacent waveguides in the arrayed waveguides 3102. When these changes are synchronized, the position at which light focuses on the terminal end of the second slab waveguide 3103 can be maintained, and a flat transmission spectrum can be obtained for the optical wavelength multiplexing/demultiplexing circuit.
To perform the above described synchronous operation for the AWG, it is required that the optical frequency channel spacing of light to be demultiplexed to the second input and output waveguides 3104 match the optical frequency spacing of the interference circuit that is connected to the first slab waveguide 3101. The optical wavelength multiplexing/demultiplexing circuit that performs the above described synchronous operation is also called a synchronized AWG.